The disclosure is related to a method for dynamic positioning of instrumented cable towed in water and also related to a system for dynamic positioning of instrumented cable towed in water.
Especially, the present invention is related to a method and system for absolute and relative dynamic positioning (DP) of marine instrumented cables towed in parallel.
A seismic instrumented cable (streamer) is an elongate cable-like structure (often up to several thousands meters long), which comprises an array of hydrophone cables and associated with electric equipment along its length, and which is used in marine seismic surveying. In order to perform a 3D/4D marine seismic survey, a plurality of such instrumented cables is towed behind a seismic survey vessel. Acoustic signals are produced by that the seismic sources are directed down through the water and into the seabed beneath, where they are reflected from the various strata. The reflected signals are received by the hydrophone cables, and next digitized and processed to form a representation of the earth strata in the area being surveyed.
The instrumented cables are typically towed at a constant depth of about five to ten meters, in order to facilitate the removal of undesired “false” reflections from the water surface. In order to keep the instrumented cables at a constant depth, control devices known as “birds” are attached to each instrumented cable at intervals of 200 to 300 meters.
Low frequency depth variations and lateral motions are inevitable. The main reason for instrumented cable depth variations is long periodic waves and changes of salinity and thus buoyancy along the cable.
In general, the worst-case situation is when towing in the same direction as the swell. Instrumented cable lateral motions are mainly due to sea current components perpendicular to the towing direction. Relatively large deviations can also appear in areas with brackish water where river course flows into the sea, something which can result in water stratification with different density. In cases of both swell and crosscurrent affection, the risk of streamer entanglement is increased.
The instrumented cable tension decreases proportionally to the distance from the towing point. Therefore, low frequency instrumented cable lateral and vertical motions tend to have larger amplitudes closer to the tail. However, the forces acting perpendicularly to the instrumented cable are non-uniformly distributed over the instrumented cable length, and change over time as the towed array moves forward.
During a seismic survey, the instrumented cables are intended to remain in a straight line, parallel to each other, equally spaced and at the same depth. However, after deployment of the instrumented cables, it is typically necessary for the vessel to cruise in a straight line for at least three instrumented cable lengths before the instrumented cable distribution is approximately close the ideal arrangement and the survey can begin. This increases the time it takes to perform the survey, and therefore increases the costs of the survey. However, because of sea currents, the instrumented cables fail to accurately follow the path of the seismic survey vessel, and sometimes deviating from this path at an angle, known as the feathering angle. This can negatively affect the covering of the survey, frequently requiring that certain parts of the survey must be repeated. In extremely unfortunate circumstances, the instrumented cables can become entangled in each other, especially at the tail of the instrumented cables, something which can cause great damages and considerable financial loss.
U.S. Pat. No. 5,790,472 (Workman and Chambers) describes a closed-loop system for controlling lateral position of seismic cables relative to their respective adjacent cables. The method is based on measured positions along the cables. If two cables come too close to each other, the control devices along the cables are commanded to set up a lateral force such that the distance between the cables is increased again. In the literature, this is known as bang-bang control strategy.
U.S. Pat. No. 6,691,038 (Zajac) describes a closed-loop system for controlling lateral position of seismic cables, either relative to their respective adjacent cables or more generally in relation to a specific reference geometry for the entire cable-spread. The controller uses continuous measurements of cable positions along the entire length and calculates desired lateral force for each control device on the cables. In addition to position measurements, used is a separate process for prediction of the behavior of the cable-spread based on data from environments and dynamics of the towing vessel. The predictions are used for calculating optimal reference curve for the total cable-spread. This is a high-level guidance functionality which must not be confused with adaptive real-time controlling.
NO 332563 (Rinnan et. al.) describes a closed-loop system for controlling lateral positioning of seismic cables where the wings of the control devices are provided with acoustic receiver and transmitter elements for measuring distance/position relative control devices on the adjacent cables. Controlling lateral position can thus be performed either locally on the control devices or globally on the towing vessel based on a telemetric model of the positions of the control devices in the seismic array. A robust controller in the control devices ensures minimal connection between the different control loops for lateral and vertical force and roll moment, respectively. Achieved is thus increased stability of the control devices, and robustness in relation to fault situations where separate wings stop to function.
A problem with the above mentioned inventions is that they either do not use a prediction model as basis for adjustment of power control output for the control devices, or that the prediction model being used is not updated as a function of varying operational conditions or external disturbances. The control system is thus not optimal with regard to response time, and is not robust in relation to unexpected incidents changing dynamics of the total system.